In an optical communication system, it is required to stabilize the operation of a semiconductor laser used as a light source.
To do so, in Patent-related Document 1, it is suggested to prevent light reflected by a wavelength filter from returning to a semiconductor laser, when monitoring oscillation wavelength of the semiconductor laser by using, via the wavelength filter, an optical detector.
In Patent-related Document 2, it is suggested to guide the light reflected by an optical resonator placed within an external modulator, toward a direction different from a direction of a semiconductor laser.
However, each of these techniques merely prevents light reflected in a semiconductor laser module from returning to a semiconductor laser source. Thus, even if an optical communication system is constructed by use of such a module, light will be reflected at positions of connection of other optical components succeeding to an optical modulator.
One method for solving the above problem is shown in Patent-related Document 3, wherein an optical isolator is placed near an outputting end of a semiconductor laser for preventing reflected light from returning to the semiconductor laser. However, since the optical isolator is an expensive optical component, this construction will increase the cost.